1. Field of the Invention
The present invention relates to an optical recording medium for recording and reading information by means of a laser beam.
2. Description of the Background
Optical recording media which are used for the recording and reading of information by means of a laser beam have rapidly become practical because of the recent developments in the fundamental technology of semiconductor lasers, recording materials and film making processes, and also because of their ability to record a large amount of information. For an optical recording medium to be able to record information, a phase change leading to an optical change should take place in the portion of the medium struck with a laser beam. Bubble (pore) making, pitting (pit or cavity formation), and amorphous state-crystalline state transfer are some of the recording methods which have been proposed so far. For example techniques for recording media utilizing bubble making are disclosed in U.S. Pat. Nos. 4,300,227 and 4,404,656.
In the case of recording media which function by pit or cavity formation (hereinafter sometimes simply referred to as "pitting"), the chief type of medium employed is one which has a recording film consisting of a thin metal film comprising as its main component a chalcogen such as Te, Se, S, or the like. The techniques which are used to form the media are disclosed, e.g., in Japanese Patent Application Laid-Open Nos. 71193/1983, 110634/1977, 146691/1982, etc. However, since most of chalcogen elements are chemically unstable, the durability of media which utilize such elements is doubtful. Attempts have been made to improve such media by incorporating various additives therein, by providing protective films, or the like. Another improvement technique is a stabilization method which involves the formation of a solid solution with a stable compound such as Te-Se alloy or the like. Still another method involves the formation of an alkylate. However, these various methods which attempt to provide media of improved properties do not completely solve the problems of the media which utilize chalcogen elements in that their recording and reading characteristics decrease with time when the media are stored over a long period. In some cases, the reflectivity tends to change depending on the thickness of the recording film, and thus it is essential to strictly specify the thickness of the film.
Further, since the semiconductor laser has a better stability with time at lower output power, which power tends to deteriorate at higher outputs, high-sensitivity optical recording media are more preferred. From the point of view of the necessity of copying with high-speed and high-density recording, demands for recording media having higher sensitivity have increased.